vfork(2) fails after unshare(CLONE_NEWTIME) (was: [Bug 215769] man 2 vfork() does not document corner case when PID == 1)

[Alejandro Colomar (man-pages)]

[Added some kernel CCs that may know what's going on]

Hi,

On 3/31/22 09:53, bugzilla-daemon@xxxxxxxxxx wrote:
> https://bugzilla.kernel.org/show_bug.cgi?id=215769
> 
> --- Comment #3 from Коренберг Марк (socketpair@xxxxxxxxx) ---
> Hi,
> I appreciate depth of information validation. Actually, you are right. vfork()
> DOES work with pid=1 processes. I figured out the cause in my case. In order to
> reproduce -- add unshare(CLONE_NEWTIME) just before vfork(). Now, I don't know
> if it's a bug in vfork() or in fork(). Yes, both are clone() actually.
> 
> In any case, they should either both give EINVAL or both don't fail. But it's
> definitely bug in the kernel around CLONE_NEWTIME.
> 

On 3/31/22 10:12, bugzilla-daemon@xxxxxxxxxx wrote:
> https://bugzilla.kernel.org/show_bug.cgi?id=215769
>
> --- Comment #4 from Коренберг Марк (socketpair@xxxxxxxxx) ---
> #define _GNU_SOURCE 1
> #include <stdio.h>
> #include <sched.h>
> #include <stdlib.h>
> #include <unistd.h>
> #include <sys/types.h>
> #include <sys/wait.h>
> #include <err.h>
>
> #ifndef CLONE_NEWTIME
> #define CLONE_NEWTIME   0x00000080
> #endif
>
> int main (void)
> {
>   if (unshare (CLONE_NEWTIME))  err (EXIT_FAILURE, "UNSHARE_NEWTIME");
>
>   pid_t pid;
>   switch (pid=vfork ())
>   {
>   case 0:
>     _exit(0);
>   case -1:
>     err(EXIT_FAILURE, "vfork BUG");
>   default:
>     waitpid(pid, NULL, 0);
>   }
>   return 0;
> }
>

I could reproduce it with the following code.  I tried
syscall(SYS_vfork) to make sure it's not a problem in the libc wrapper,
and to make sure I do call vfork(2).  If I replace vfork(2) with
fork(2), I don't get the error.


$ cat vfork.c
#define _GNU_SOURCE
#include <err.h>
#include <linux/sched.h>
#include <sched.h>
#include <signal.h>
#include <stdlib.h>
#include <sys/syscall.h>
#include <unistd.h>

int main(void)
{
	pid_t pid;

	if (unshare(CLONE_NEWTIME) == -1)
		err(EXIT_FAILURE, "unshare(2)");
	if (signal(SIGCHLD, SIG_IGN) == SIG_ERR)
		err(EXIT_FAILURE, "sigaction(2)");
	pid = syscall(SYS_vfork);
	switch (pid) {
	case 0:
		errx(EXIT_SUCCESS, "Grandchild exiting normally.");
	case -1:
		/* If we got here, the report is confirmed. */
		err(EXIT_FAILURE, "vfork(2)");
	default:
		errx(EXIT_SUCCESS, "Child exiting normally.");
	}
}

$ cc -Wall -Wextra -Werror vfork.c
$ sudo ./a.out
a.out: vfork(2): Invalid argument



$ grep_syscall_def vfork
kernel/fork.c:2711:
SYSCALL_DEFINE0(vfork)
{
	struct kernel_clone_args args = {
		.flags		= CLONE_VFORK | CLONE_VM,
		.exit_signal	= SIGCHLD,
	};

	return kernel_clone(&args);
}


Maybe someone in the kernel can send some patch for the clone(2) and/or
vfork(2) manual pages that explains the reason (if it's intended).


Thanks,

Alex

-- 
Alejandro Colomar
Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/
http://www.alejandro-colomar.es/